Fixed MPIE being cleared leading to have interrupts being disable after an mret instruction

[cpdpls]

Fixed #517

1. MPIE was being cleared alongside MIE before issuing an mret instruction. As a side effect, this led to have no interruptions enabled forever when returning from the very first context switching.
2. Removed some assembly leftovers from the previous ports when changing the mstatus register inside the _tx_thread_context_restore routine

PR checklist

• Updated function header with a short description and version number
• Added test case for bug fix or new feature
• Validated on real hardware (Microchip Polarfire SOC -- riscv64-unknown-elf-gcc (xPack GNU RISC-V Embedded GCC (Microsemi SoftConsole build), 64-bit) 8.3.0